Sintered material

ABSTRACT

A sintered material made of an iron-based alloy is provided, wherein a content of Ni is more than 0.2 mass % and 10 mass % or less in an entire iron-based alloy; a content of C is more than 0 mass % and 2.0 mass % or less in the entire iron-based alloy; at least one element selected from Mo, Mn, Cr B and Si is more than 0 mass % and 5.0 mass % or less in total in the entire iron-based alloy; and a rest of the iron-based alloy is Fe and incidental impurities. A content of Ni in a local region of the iron-based alloy is more than 0.2 mass % and less than 21 mass %. A relative density is 97% or more.

TECHNICAL FIELD

The present disclosure relates to a sintered material.

The present application is based on and claims priority to Japanese Patent Application No. 2017-144801, filed on Jul. 26, 2017, the entire contents of which are herein incorporated by reference.

BACKGROUND ART

Patent Document 1 discloses an iron-based sintered body containing Ni, Mo, Mn, and C within a specific range, the remainder having a composition consisting of Fe, and Ni-rich martensite interspersed in tissue made of tempered martensite.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Laid-Open Patent Application Publication No. 11-246951

SUMMARY OF THE INVENTION

A sintered material of the present disclosure is a sintered material made of an iron-based alloy,

wherein a content of Ni is more than 0.2 mass % and 10 mass % or less in an entire iron-based alloy; a content of C is more than 0 mass % and 2.0 mass % or less in the entire iron-based alloy; at least one element selected from Mo, Mn, Cr B and Si is more than 0 mass % and 5.0 mass % or less in total in the entire iron-based alloy; and a rest of the iron-based alloy is Fe and incidental impurities,

wherein a content of Ni in a local region of the iron-based alloy is more than 0.2 mass % and less than 21 mass %, and

wherein a relative density is 97% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between relative density and tensile strength of a sintered material in Test Example 1;

FIG. 2 is a graph showing a relationship between an amount of Ni in a sintered material and tensile strength in Test Example 2; and

FIG. 3 is a graph showing a relationship between an oxygen content and tensile strength of a sintered material in Test Example 3.

MODE OF CARRYING OUT THE INVENTION Problems to Be Solved by the Disclosure

A sintered material having a higher static strength and a superior fatigue strength is desired.

In an iron-based sintered body disclosed in Patent Document 1, the tensile strength is only about 1400 MPa, and it is desirable to further improve the static strength. Particularly, a sintered material having strength equal to or greater than that of a solidified member is preferable.

In addition to the tensile strength, it is also desirable for the sintered material to have excellent dynamic strength, such as fatigue strength, which is difficult to break even when the sintered material is repeatedly bent.

Therefore, one of the objectives is to provide a sintered material having high static strength and excellent fatigue strength.

Description of Embodiments of the Present Disclosure

To begin with, embodiments of the present disclosure will be listed and described below.

(1) A sintered material of one embodiment of the present disclosure is a sintered material made of an iron-based alloy,

wherein a content of Ni is more than 0.2 mass % and 10 mass % or less in an entire iron-based alloy; a content of C is more than 0 mass % and 2.0 mass % or less in the entire iron-based alloy; at least one element selected from Mo, Mn, Cr B and Si is more than 0 mass % and 5.0 mass % or less in total in the entire iron-based alloy; and a rest of the iron-based alloy is Fe and incidental impurities,

wherein a content of Ni in a local region of the iron-based alloy is more than 0.2 mass % and less than 21 mass %, and

wherein a relative density is 97% or more.

In the present disclosure, “a Ni content in a local region of an iron-based alloy is greater than 0.2 mass % but less than 21 mass %” means as follows.

A cross section of a sintered material is taken, and a measurement field having a predetermined size is taken from the cross section. The Ni content in this measurement field is measured with a SEM-EDX device. The maximum Ni content in the measurement field is less than 21 mass %, and the minimum Ni content is more than 0.2 mass %. The measurement method will be described in detail below.

Because the above-described sintered material has a high relative density of 97% or more and a high density, the above-described sintered material has high tensile strength and excellent static strength compared to a sintered material having a relative density of less than 97%. In addition, the above-described sintered material has an Ni concentration distribution satisfying the above-described specific range, and it can be said that Ni is uniformly distributed compared to the sintered material containing non-uniformly distributed Ni due to the presence of the above-described Ni-rich martensite. The aforementioned sintered material is not only difficult to break when pulled, but also difficult to crack even when repeatedly bent, and has excellent fatigue strength. One possible reason for this is as follows.

Conventionally, the sintered material is considered preferable to have a higher relative density and a higher density to increase the tensile strength of the sintered material. This is because a cavity is likely to become a starting point for cracking and fracture. The Ni-rich martensite described above around the cavity is considered to be able to locally enhance the mechanical properties of the material around the cavity and to reduce the cavity of becoming the origin of a breakage and a crack. However, as a result of the investigation by the inventors, the inventors have found that the tensile strength of a dense sintered material having a relative density of 97% or more is reduced by a composition in which Ni is non-uniformly present with respect to the entire sintered material, such as the above-described Ni-rich martensite. The above-described composition non-uniformly containing Ni contains a location containing the Ni content a lot, but also contains a location containing the Ni a little, which is considered to cause a decrease in strength.

On the other hand, when quenching and tempering are performed after sintering, the strength is particularly increased compared to a case of not performing both quenching and tempering or a case of performing only quenching, thereby acquiring the tensile strength and the fatigue strength in a balanced manner. Because the above-described low Ni concentration location is inferior in the quenching properties, the low Ni concentration location does not seem to convert to the martensite when the quenching is performed, and is likely to become residual austenites with lower strength, which may be the starting point for a breakage and a fracture.

In contrast, if the relative density is 97% or more and the Ni concentration distribution satisfies the above-mentioned specific range, a decrease in strength due to the non-uniform presence of Ni is unlikely to occur, and the sintered material is considered to excel in static strength and fatigue strength. When performing quenching and tempering, the uniform presence of Ni allows the entire Ni to be transformed into the (tempered) martensite, with less residual austenite, preferably substantially nonexistent, and the sintered material is considered to excel more in static strength and fatigue strength.

(2) As a form of the above-described sintered material, a form in which the content of Ni in the entire iron-based alloy is more than 2 mass % and less than 8 mass % is cited.

The above form has higher tensile strength, better static strength, and better fatigue strength because the Ni content satisfies the specific range described above.

(3) As a form of the above-described sintered material, a form in which a content of oxygen is less than 3000 mass ppm is cited.

The present inventors investigated the fracture surface of a dense sintered material having a relative density of 97% or more by conducting a tensile test, and found that there was an oxide on the fracture surface and that this oxide could be the starting point of the fracture. In the above-described form, because the oxygen content satisfies the above-described specific range and oxygen is low, the oxide that may be the starting point of the breakage or the fracture is likely to be reduced. Thus, the above form has a higher tensile strength, a better static strength, and a better fatigue strength.

(4) As a form of the above-described sintered material, a form in which tissue is made of martensite is cited.

The above-described form is typically subjected to quenching and tempering after sintering. Such a form has higher tensile strength, better static strength, and better fatigue strength than a form without quenching and tempering.

Details of Embodiments of the Present Disclosure

Hereinafter, an embodiment of the present disclosure will be described in detail. In the following description, the content of an element represents the mass percentage (mass % or mass ppm).

Embodiment Sintered Material

A sintered material according to an embodiment is formed by binding a plurality of metal particles made of an iron-based alloy having Fe as a main body, and has very few cavities and a high density. Specifically, the sintered material according to the embodiment is a sintered material made of an iron-based alloy, wherein the content of Ni in the entire iron-based alloy is more than 0.2 mass % and 10 mass % or less; the content of C is more than 0 mass % and 2.0 mass % or more; at least one element selected from Mo, Mn, Cr, B, and Si is more than 0 mass % and 5.0 mass % or less with respect to the entire iron-based alloy; the rest is Fe and incidental impurities; and a relative density is 97% or more.

In particular, in the sintered material of an embodiment, Ni is present uniformly. Specifically, the sintered material in an embodiment has a Ni content in a local region of an iron-based alloy more than 0.2 mass % and less than 21 mass %. More detailed description is as follows.

Overall Composition

In addition to Fe, the sintered material of the embodiment contains elements such as Ni, C and Mo having the strength enhancement effect, and thus has excellent strength.

The entire iron-based alloy contains Ni in the range of 10% or less, which has excellent hardenability, and reduces residual austenite when quenched and tempered, thus easily having martensitic tissue. Therefore, the mechanical properties of the sintered material are easily improved. When the Ni content is 1% or more, the tensile strength can be improved, and the Ni content is preferably 2% or more. If a further increase in strength is desired, the content of Ni is preferably more than 2% and less than 8%, and if the content is 2.5% or more and 7.5% or less, 3% or more and 7% or less, and 4% or more and 6% or less, the sintered material is likely to have higher strength more and more.

The sintered material excels in strength by containing C in the range of 2.0% or less. In particular, if the content of C is 0.1% or more and 1.5% or less, further 0.2% or more and 1.0% or less, and still further 0.2% or more and 0.8% or less, the sintered material is likely to have higher strength more and more.

The sintered material excels in strength by containing elements such as Mo in a total range of 5.0% or less. In particular, if the total content of these elements is 0.1% or more and 3.0% or less, and further 0.2% or more and 2.0% or less, the sintered material readily has higher strength more and more. Also, in particular, when at least one of Mo and Mn, and preferably both, is included, the sintered material is likely to have higher strength more and more. The content of Mo and the content of Mn are 0.1% to 1.0% and 0.15% to 0.8%, respectively.

For the measurement of the overall composition of the sintered material, for example, an inductively coupled plasma optical emission spectrometry (ICP-OES) may be used.

Distribution of Ni Concentration in Local Region (Content)

In the sintered material of the embodiment, the content of Ni in the local region of the iron-based alloy (hereafter referred to as a “Ni concentration distribution”) is greater than 0.2 mass % and less than 21 mass %. That is, there is virtually neither a location where the Ni content is 0.2% or less (hereinafter referred to as a “low nickel region”) nor a location where the Ni content is 21% or more (hereinafter referred to as a “high Ni region”). In the low Ni region, because the Ni content is too low, the hardenability is particularly poor, and Ni is likely to be present as residual austenite when quenched and tempered. The high Ni region readily stabilizes austenite because of the too much Ni content, and is unlikely to convert to martensite when quenched and tempered. As with the low Ni region described above, the high Ni region is likely to be present as residual austenite. That is, in the sintered material including a low Ni region and a high Ni region, the (residual) austenite is locally present even after quenched and tempered, which is inferior in tensile strength and fatigue strength.

The smaller the range of the Ni concentration distribution, that is, the smaller the difference between the maximum value and the minimum value of the Ni content, the more uniformly Ni present, which is likely to reduce the low Ni region that can be the starting point of the breakage and the fracture. The Ni concentration distribution is preferably 0.3% or more and 20% or less, further preferably 0.4% or more and 18% or less, 0.5% or more and 16% or less, and 1% or more and 12% or less. More preferably, the range of the Ni concentration distribution (the difference above) is substantially zero. In this case, the content of Ni in the overall composition of the sintered material is substantially equal to the above-described maximum value and minimum value in the Ni concentration distribution.

Oxygen Content

The sintered material according to the embodiment can reduce the oxide that may be the starting point of the breakage or fracture when the oxygen content is further low, and is superior in tensile strength and fatigue strength, which is preferred. Quantitatively, the oxygen content is preferably less than 3000 ppm, and more preferably 2500 ppm or less, and even more preferably 2000 ppm or less.

Tissue

The sintered material of the embodiment may remain sintered, but is more superior in tensile strength and fatigue strength, which is preferable, when quenched and tempered. In this case, the sintered material of the embodiment has tissue composed of (tempered) martensite. In particular, because the sintered material of the embodiment uniformly contains Ni as described above, the entire sintered material is susceptible to transformation to martensite, and the local presence of residual austenite can be reduced. Preferably, the entire sintered material is substantially comprised of martensite, and the tissue is substantially free of residual austenite.

Density

The sintered material according to the embodiment has a relatively high density of 97% or more in addition to a uniform Ni content as described above. Because the cavity of the sintered material is very few, a breakage and a fracture caused by the cavity are unlikely to occur, and the sintered material has high strength. The relative density may be 97.5% or more, 98% or more, or 98.5% or more.

The relative density (%) of a sintered material may be obtained, for example, by (apparent density of the sintered material/true density of the sintered material)*100. The apparent density of the sintered material may be obtained, for example, in accordance with the Alchimedes method. The details are described below.

Alternatively, the relative density (%) of the sintered material may be obtained by analyzing the cross section of the sintered material using commercially available image analyzing software. Specifically, in the cross section of the sintered material, images of a plurality of fields of view are acquired, and the plurality of fields of view is observed (for example, n≥10). Any cross section may be taken as the cross section. Multiple cross sections may be taken by taking one field of view per cross section, or multiple fields of view may be taken per cross section. The size of each field of view is made 500 μm*600 μm. The image of each field of view is binarized to obtain a percentage of a metal area with respect to each field of view, and the percentage of the metal area is regarded as the relative density of each field of view. Then, the relative densities of the plurality of fields of view are averaged and the average value is defined as the relative density of the sintered material.

Mechanical Characteristics

The sintered material of an embodiment has high tensile strength and excellent static strength because of the uniform inclusion of Ni in addition to the dense properties as described above. Quantitatively, the tensile strength is, for example, 1455 MPa or more, further 1460 MPa or more, 1500 MPa or more, 1550 MPa or more, 1580 MPa or more, or 1600 MPa or more. The tensile strength is likely to be higher when at least one of the following conditions is satisfied: the above relative density is as high as possible (Example 1, described below); the range of Ni concentration distributions is as small as possible (Example 1, described below); the Ni content is close to 5% (Example 2, described below); and the oxygen content is as low as possible (Example 3, described below).

Intended Purpose

The sintered material of the embodiment is suitably utilized in a variety of general structural components, for example, mechanical components such as sprockets, rotors, gears, rings, flanges, pulleys, and bearings.

Method of Manufacturing Sintered Material

The sintered material of an embodiment includes, for example, a process of preparing a raw material powder, a process of producing a powder compact by pressurizing the raw material powder, and a process of sintering the powder compact to form a sintered material. Further, for example, a process of producing a heat-treated material by quenching and tempering the sintered material may be performed. Each process will be described in detail below.

Raw Material Preparation Process

In this process, a raw material powder containing an iron-based powder having a plurality of iron-based particles is prepared. The “iron-based” refers to pure iron or an iron alloy mainly composed of iron. The raw material powder includes any one of (1) a mixed powder containing Ni as a powder, (2) an iron alloy powder containing Ni as an additive element, and (3) a compound powder containing both a mixed powder and an iron alloy powder. When the raw material powder contains the iron alloy powder, because the iron-based powder itself uniformly contains Ni, the above-described Ni concentration distribution facilitates the production of the sintered material according to the embodiment that satisfies the specified range, and the sintered material is considered suitable for commercial mass production.

(1) The mixed powder typically contains pure iron powder, Ni powder, C powder, and a powder of one or more elements selected from Mo, Mn, Cr, B and Si. The proportion of each powder may be adjusted to obtain the sintered material having a desired composition, where the content of elements such as Ni, C, Mo and the like satisfies the above-described range. This point is also true for (3) a composite powder, which will be discussed later.

(2) Typically, the iron alloy powder includes a Fe-Ni based alloy powder containing Fe as the main component and containing Ni and elements such as Mo as described above. The content of elements such as Ni and Mo in the Fe-Ni based alloy may be adjusted to obtain a sintered material of the desired composition (where the content of elements such as Ni, Mo and the like satisfies the above-described range). When an iron alloy powder is used, C (carbon) is not contained as an element to be added to the iron alloy, but is contained in the raw material powder as an independent powder (C powder).

(3) The composite powder typically contains pure iron powder, Ni powder, iron alloy powder containing Ni, and C powder. When the composite powder is used, the proportion is adjusted so that the total content of Ni in the powder and the powder of iron alloy containing Ni satisfies the above-mentioned range (10% or less).

The iron-based powders include water-atomized powder, reducing powder, gas-atomized powder, and carbonyl powder. The average particle size of the iron-based powder is, for example, 20 μm or more and 200 μm or less. If the average particle diameter is within the above-described range, the iron-based powder can be easily handled and formed by being pressed. In addition, if the average particle diameter is 20 μm or more, the flowability of the iron-based powder is likely to be ensured, and the iron-based powder is excellent in formability. If the average particle diameter is 200 μm or less, a sintered material of dense tissue is likely to be obtained. The average particle size may be 50 μm or more and 150 μm or less.

The average particle size of the powder of elements such as Ni powder and Mo powder is, for example, not less than 1 μm and not more than 50 μm. For example, the average particle diameter of C powder may be about 1 μm or more and about 30 μm or less, and C powder that is smaller than the iron-based powder may be used.

The above-described average particle size is defined as the particle size (D50) in which the cumulative volume in the volume particle size distribution measured by a laser diffraction particle size distribution measuring device is 50%.

The raw material powder can contain at least one of a lubricant and an organic binder. In this case, if the total content of the lubricant and the organic binder is 0.1% or less, a dense powder compact is likely to be obtained, which is preferable. The absence of the lubricant and the organic binder makes it easier to obtain the dense powder compact, and degreasing the compact is not required in subsequent processes.

Molding Process

In this process, it is preferable that the raw material powder be pressed to form a powder compact having a relative density of 96% or more and further 97% or more. This is because a sintered material having a relative density of 97% or more can be obtained more reliably. As the relative density of the powder compact becomes higher, the denser sintered material having the higher relative density can be readily obtained. Thus, the relative density of the powder compact is 98% or more and further 99% or more.

The shape of the powder compact may include a shape along the final shape of the sintered material or a shape suitable for a cutting process in the post-process (for example, columnar shape or cylindrical shape). Production of the powder compact includes the use of a suitable molding apparatus capable of forming the above shape. Particularly, the use of a press molding apparatus capable of uniaxial pressing along the axial direction of the column or the cylinder facilitates obtaining the powder compact as described above. Uniaxial pressing includes using a mold comprising a die having an aperture above and below, and an upper and lower punch fitted into the aperture above and below the die. The die cavity in the mold is filled with a raw material powder and the raw material powder in the cavity is compressed with an upper punch and a lower punch to create a powder compact.

If the molding pressure (surface pressure) is 1560 MPa (approximately 16 tons/cm²) or more, the powder compact as described above can be produced. The higher the molding pressure, the higher the relative density of the powder compact, and the molding pressure can be made 1660 MPa (≈17 ton/cm²) or more, 1760 MPa (≈18 ton/cm²) or more, 1860 MPa (≈19 ton/cm²) or more, and 1960 MPa (>20 ton/cm²) or more. When the raw material powder contains the aforementioned iron alloy powder, if the molding pressure is made high, the powder compact excels in moldability.

When the lubricant is applied to the inner peripheral surface of the above-described mold (the inner peripheral surface of the above-described die or the pressing surface of the punch), it is possible to prevent the raw material powder from burning onto the mold, and the dense powder compact can be easily fowled, which is preferable. For example, a higher fatty acid, a metal stone, a fatty acid amide, or a higher fatty acid amide can be used as the lubricant.

Sintering Process

In this process, the powder compact is sintered, thereby producing a sintered material having a relative density of 97% or more and a Ni concentration distribution that satisfies the specified range described above. Because the powder compact shrinks when sintered, if the relative density of the powder compact is 96% or more and 97% or more as described above, the sintered material having the relative density of 97% or more can be more reliably produced. If the relative density of the powder compact is very high as described above, the relative density of the sintered material may exceed the relative density of the powder compact, although the shrinkage amount while being sintered is small.

The sintering conditions may be selected depending on the composition of the raw material powder.

Examples of sintering temperatures include 1100 degrees C. or more and 1400 degrees C. or less, 1110 degrees C. or more and 1300 degrees C. or less, and 1120 degrees C. or more and 1250 degrees C. or less.

For example, the sintering period of time may be 15 minutes or more and 150 minutes or less, or 20 minutes or more and 60 minutes or less.

The atmosphere while sintering includes an inert atmosphere such as a nitrogen atmosphere.

Other sintering conditions may refer to known conditions.

Other Processes

After the sintering process, at least one of the following of a molding process, a heat treatment process, and a finishing process is performed.

Molding Process

This process involves cutting the powder compact after the above-described molding process and before the sintering process. The cutting process may be performed with a suitable cutting tool depending on the cutting contents. If the pre-sintered powder compact is machined, it is easier to be cut than a sintered material or a wrought steel. In particular, although the powder compact is softer than a sintered material or a wrought steel, because the powder compact has a high relative density and a high density as described above, and has a certain degree of strength, it is easy to prevent chips and cracks from occurring due to the cutting process. The cutting processes include, for example, a rolling (including drilling) process, a turning process and the like.

Before the cutting process, if a volatile solution containing an organic binder (for example, paraffin, various waxes and the like) or a thermoplastic solution such as polyethylene is applied to the surface of the powder compact, or the powder compact is immersed in the solution, the surface layer of the powder compact can be readily prevented from breaking or chipping during the cutting process.

In addition, when the cutting process is performed while applying a compressive stress to the powder compact in the direction in which the tensile stress acting on the powder compact is eliminated, cracking or chipping of the powder compact is readily prevented.

Heat Treatment Process

The sintered material is quenched and tempered in this process. The martensitic structure is formed by quenching, and the martensitic structure is stabilized by tempering. The quenching and tempering, in particular, improves hardness and toughness and can be a sintered material with superior mechanical properties as compared to the sintered state. In particular, because the sintered material before quenching and tempering has a Ni concentration distribution of more than 0.2% and less than 21%, as described above, the heat treated material after quenching and tempering (an example of a sintered material in accordance with the embodiment) can reduce the residual austenite, and the entire heat treated material can be more reliably made the sintered material consisting of a martensitic structure (tempered martensitic structure).

Typically, quenching involves carburizing.

Carburizing conditions include carbon potential (C. P.) of 0.8 mass % to 1.4 mass %, a treatment temperature of 910 degrees C. to 950 degrees C., and a treatment period of time of 60 minutes to 150 minutes.

Austenitization conditions include a treatment temperature of 850 degrees C. to 1000 degrees C., a treatment period of time of 10 minutes to 150 minutes, and then quenching with oil cooling or water cooling.

The tempering conditions include a treatment temperature of 150 degrees C. to 230 degrees C. and a treatment period of time of 60 minutes to 150 minutes.

The Ni concentration distribution of the sintered material before quenching and tempering does not substantially change with quenching and tempering. Thus, the Ni concentration distribution in the heat treated material after quenched and tempered takes the same range as the Ni concentration distribution in the sintered material before quenched and tempered, that is, more than 0.2% and less than 21%.

Finishing Process

In this process, the sintered material is processed to reduce the surface roughness of the sintered material and to fit the dimension of the sintered material to the designed dimension. The finishing process includes, for example, polishing.

The sintered material produced by the above-described sintered material manufacturing method is substantially uniform in density at its surface area (typically 1 mm thick from the surface to the interior). This is because the sintered material is not rolled. Further, the metal structure of the sintered material does not have a streamlined structure in which the metal particles are stretched. This is because the sintered material is not forged.

Major Effects

Because the sintered material of embodiments has a very high relative density and a dense structure, and uniformly contains Ni, locations where a breakage and a fracture can occur are few. Thus, the sintered material of the embodiment excels in static strength and fatigue strength. The high strength will be specifically described in the following test examples.

TEST EXAMPLE 1

Sintered materials having various relative densities were made, and the relationship between the relative density and the tensile strength was examined.

In this test, a mixed powder (raw material No. 2), an iron alloy powder (raw material No. 1), and a composite powder containing a mixed powder and an iron alloy powder with a different proportion of nickel powder (raw material No.31, 32, 33) are prepared as the raw powder, and powder compacts having different relative densities are made using each raw material powder. The relative density of the compact is selected from a range of about 91% to about 99%. The molding pressure is selected from a range of 1560 MPa (16 tons/cm²) to 1960 MPa (20 tons/cm²) to obtain a powder compact having a predetermined relative density. The higher the molding pressure, the easier obtaining a powder compact having a higher relative density. In addition, the higher the relative density of the powder compact, the easier obtaining a sintered material having a higher relative density.

Each raw material powder is used by adjusting the mixing percentage to satisfy the basic composition of Fe—5 mass % Ni—0.5 mass % Mo—0.2 mass % Mn—0.3 mass % C. In this test, each raw material powder contains neither a lubricant nor an organic binder (no internal lubrication).

The powder of the raw material No. 1 is a mixture of pure iron powder, pure nickel powder, pure Mo powder, pure Mn powder, and pure C powder.

The powder of the raw material No. 2 is a mixture of the iron alloy powder of Fe—5 mass % Ni—0.5 mass % Mo—0.2 mass % Mn and pure C powder.

The powder of the raw material No. 31 is a mixture of the iron alloy powder of Fe—3 mass % Ni—0.5 mass % Mo—0.2 mass % Mn, pure Ni powder, and pure C powder.

The powder of the raw material No. 32 is a mixture of the iron alloy powder of Fe—2 mass % Ni—0.5 mass % Mo—0.2 mass % Mn, pure Ni powder, and pure C powder.

The powder of the raw material No. 33 is a mixture of the iron alloy powder of Fe—0.5 mass % Ni—0.5 mass % Mo—0.2 mass % Mn, pure Ni powder, and pure C powder.

Here, metal powders such as pure iron powder, iron alloy powder, Ni powder, Mo powder, and Mn powder are all produced by known methods such as a water atomizing method. The average particle size (D50) of pure iron powder is 75 μm, the average particle size (D50) of iron alloy powder is 70 μm, the average particle size (D50) of Ni powder is 5 μm, the average particle size (D50) of Mo powder and Mn powder is 10 μm, and the average particle size (D50) of C powder is 5 μm. The above-described metal powder is appropriately subjected to a reduction treatment and the like to decrease the oxygen content.

The raw material powder is pressurized to form a columnar powder compact. To produce the powder compact, a die capable of uniaxial pressing is used. An alcohol solution of myristic acid is applied to the inner periphery of the die in this die as a lubricant (with external lubrication).

Each produced compressed powder compact is sintered, and the resulting cylindrical sintered material is subject to a cutting process to form a prescribed tensile test piece, which is subsequently subjected to a heat treatment, and the obtained heat treated material is used as the sintered material of each sample. Here, the heat treatment is carburized quenching and tempering. The sintering, carburizing, and tempering conditions are as follows.

(Sintering) 1130 degrees C.*30 minutes in a nitrogen atmosphere

(Carburizing) 930 degrees C.*90 minutes, carbon potential: 1.2 mass %->850 degrees C.*30 minutes->oil cooling

(Tempering) 200 degrees C.*90 minutes

A relative density and tensile strength of a sintered material of each produced sample were measured. The results are shown in Tables 1 and FIG. 1.

In Table 1, sample numbers are given for each relative density. In the following description, each sintered material is called by a number that combines the sample number with the raw material number. For example, “Sample No. 5-1” means a sintered material made of the powder of the raw material No. 1 with a relative density of 99%.

FIG. 1 is a graph in which the horizontal axis indicates the relative density (%) of the sintered material and the vertical axis indicates the tensile strength (MPa) of the sintered material. FIG. 1 also shows the tensile strength of a wrought steel (100% relative density) to be described later.

The relative density of the sintered material is obtained by (apparent density of sintered material/true density of sintered material)*100. The apparent density of the sintered material is obtained in accordance with the Alchimedes method. Specifically, the mass in air and the mass in pure water of the sintered material are measured, and the apparent density of the sintered material is calculated by “(Density of pure water*Mass in air)/(Mass in air−Mass in pure water)”.

The true density of a sintered material may be calculated by analyzing the components of the sintered material by, for example, ICP-OES to obtain the content of each element, and using the content ratio, the density of each element, and the mass of the sintered material. In this test, the true density of the sintered material can be obtained from the basic composition of the raw material powder. Here, the true density of the sintered material is 7.82 g/cm³.

The tensile strength is measured by conducting a tensile test using a general-purpose tensile testing machine. The specimen has a flat plate shape composed of a narrow width piece and wide width pieces formed at both ends of the narrow width piece. The specimen is 5 mm thick and 72 mm long. The narrow width piece consists of a central portion and a shoulder having an arcuate side surface formed from the central portion to the wide width piece. The center length is made 32 mm; the center width is made 5.7 mm; both ends' width is made 5.96 mm; the radius of the side surface of the shoulder is made 25 mm, and the width of the wide width piece is made 8.7 mm. These test pieces conform to the standards of the Japan Powder Metallurgy Manufacturers Association, JPMA M 04-1992 Sintered Metal Material Tensile Test Piece.

For comparison, a wrought steel having the above-described basic composition (relative density: 100%) was prepared; the above-described test pieces were produced; the tensile strength was measured; and 1695 MPa was obtained.

For a sintered material of each produced sample, the local Ni concentration distribution (content) was measured as follows.

A plurality of any cross sections is taken with respect to the sintered materials of Samples No. 5-1, No. 5-31 to 33, and No. 5-2 having a relative density of 99% (n≥3). In addition, one measurement field (400 μm*500 μm) is taken for each cross section. The Ni content in each measurement field is measured with a SEM-EDX device, and the maximum value and the minimum value of the Ni content in each measurement field are examined. The electron beam used in the SEM-EDX device has a radius of about 5 μm. That is, the spatial resolution is about 5 μm ϕ. Among the plurality of maximum values and minimum values (n≥3) of each of the measurement fields, the higher maximum value and the lowest minimum value of each of the measurement fields are shown in Table 2 as the maximum value and the minimum value of Ni in the sintered material of each sample.

Incidentally, the Ni concentration distribution in the sintered material having the relative density of 91% to 97% is almost the same as the Ni concentration distribution having the same raw material number in the sintered material having the relative density of 99%. For example, the Ni concentration distribution in the sintered material of Sample No. 3-33, which is produced using the powder of the raw material No. 33 and has a relative density of 95%, is approximately similar to the Ni concentration distribution of Sample No. 5-33.

Otherwise, when the oxygen content of each sintered material of the produced sample was measured, any of the oxygen contents was less than or equal to 2000 mass ppm. To measure the oxygen content, a melting infrared absorption method that extracts oxygen by heating and melting a sample in an inert gas and measures extracted oxygen, is used. Commercial oxygen analyzers can be used for this measurement. The overall composition of the sintered material of each sample was measured by ICP-OES, and the basic composition of the raw material powder was almost the same as that of the above-described material powder.

TABLE 1 SAMPLE RELATIVE MATERIAL No. No. DENSITY (%) 1 31 32 33 2 1 91 1022 — — 1040 1080 2 93 1246 — — 1340 1298 3 95 1492 — — 1540 1418 4 97 1692 — — 1597 1455 5 99 1696 1675 1643 1599 1442 *TENSILE STRENGTH (MPa)

TABLE 2 MATERIAL No. 1 31 32 33 2 AFTER HEAT MINIMUM 5 3 2 0.5 0.2 TREATMENT VALUE Ni AMOUNT MAXIMUM 5 8 10 16 21 (MASS %) VALUE

TABLE 1 and FIG. 1 indicate that in the range where the relative density is less than 97%, the higher the relative density, the higher the tensile strength, and that the tensile strength proportionally increases. However, when the relative density is 97% or more, it can be seen that the change in tensile strength is very small even when the relative density is higher with respect to Sample having the raw material number. Thus, the result indicates that it is difficult to further improve the tensile strength by increasing the relative density of the sintered material having the relative density of 97% or more. Cracking and fracture are considered to occur in the sintered materials with a relative density of 97% or more due to a factor other than a cavity, because it is difficult to increase the relative density and to improve the tensile strength even with fewer cavities. The difference in the manufacturing conditions, especially the difference in the raw material powder is considered to be the factor other than the cavity from TABLE 1 and FIG. 1.

The difference in manufacturing conditions may cause a difference in the composition and tissue of the sintered material. Thus, the tissue of sample No. 5-2 having a tensile strength of less than 1450 MPa despite having a high relative density of 99% was examined. A tissue analysis of the sintered material of sample No. 5-2 using mixed powder as the raw material powder was performed using a SEM-EBSD device. The nickel content at this analysis point was measured using the SEM-EDX device, and a nickel mapping image was obtained. In the Ni mapping image, the darker the color (the darker the monochrome image), the lower the nickel content, and the lighter the color (the brighter the monochrome image), the higher the nickel content.

In the histological analysis, a green region (thin gray region in monochrome images) is a face-centered cubic lattice (fcc), and the other red region (dark gray region in monochrome images) is generally the body-centered cubic lattice (bcc). The fcc region is residual austenite and the bcc region is martensite. The fcc region appears bright in the Ni mapping image, indicating that the nickel content is locally high. In contrast, the bcc region appears dark in the Ni mapping image, with locally low nickel content. That is, the sintered material of Sample No.5-2 is non-uniform in Ni. It is considered probable that this non-uniform composition is likely to cause a decrease in strength, and that when quenched and tempered, the sintered material is likely to include residual austenite inferior in strength, and the tensile strength is likely to further decrease.

Based on the above findings, the sintered material of Samples No. 4-1, No. 4-33, No. 5-1, No. 5-31 to No. 5-33 (hereinafter referred to as a “uniform sample group”) having the relative density of 97% or more and the Ni concentration distribution more than 0.2 mass % and less than 21 mass % has a high tensile strength of 1460 MPa or more, and a high tensile strength of 1500 MPa or more and 1550 MPa or more, which are excellent in static strength. In particular, each of the sintered materials of Samples No. 4-1 and 5-1 has a tensile strength of 1692 MPa or more and has a strength equal to or greater than that of the solidified member (1695 MPa).

One reason for the high strength of the sintered material in the homogeneous sample group as described above is the uniform presence of Ni throughout the sintered material. This is supported by comparing Sample No. 5 having 99% relative density to the raw material No. 1, 31 to 33, and 2. In the order of Samples No. 5-1, No. 5-31 to No. 5-33, and No. 5-2, the range of the Ni concentration distribution is small, and the difference between the maximum value and the minimum value in Ni amount is small. Specifically, in Sample No. 5-1, there is substantially no difference as described above, and Ni is uniformly present throughout the sintered material. In the order of Samples No. 5-31 to No. 5-33, the range was 3 mass % to 8 mass % and the difference was 5 mass %, the above-mentioned range was 2 mass % to 10 mass % and the above-mentioned difference was 8% by mass, the above-mentioned range was 0.5 mass % to 16 mass %, and the above-mentioned difference was more than 10 mass %. In Sample No. 5-2, the above-mentioned difference is 20 mass % or more, while indicating that the nickel is present non-uniformly.

The cross section of sample No. 5-1 using an iron alloy powder and Sample No. 5-2 using a mixed powder were observed with SEM, and the content of Ni at this observation point was measured with the SEM-EDX device. In the sintered material of Sample No. 5-1, the entire Ni mapping image was dark, and Ni was found to be uniformly present. Based on the Ni distribution, the sintered material of Sample No. 5-1 is considered to be substantially free of residual austenite and to have martensitic tissue throughout thereof. Based on this, the sintered material having the relative density of 97% or more and Sample No. 5-31 to No. 5-33 having the small Ni concentration distribution as described above is considered to have a martensitic structure, although the sintered material may contain some residual austenite.

On the other hand, in the sintered material of Sample No. 5-2, some areas seemed bright and some seemed dark, indicating that Ni was found to be unevenly present as a whole.

As described above, this test showed that the sintered material with a relative density of 97% or more has a higher strength, preferably strength as approximately strong as that of the wrought steel having the same composition as that of the sintered material, and excels in strength by setting the Ni concentration distribution to more than 0.2 mass % and less than 21 mass %.

From this test, when the sintered material having the relative density of 97% or more and the Ni concentration distribution of more than 0.2 mass % and less than 21 mass % contains iron alloy powder (in this case, using the raw materials Nos. 1 and 31 to 33), the sintered material is readily manufactured. In particular, it is preferable to use the powder of the raw material No.1, that is, preferable to mainly use the powdered iron alloy.

TEST EXAMPLE 2

The effect on tensile strength was examined by varying the nickel content.

Here, the sintered material is produced in the same manner as Test Example 1, except that the content of Ni in the iron alloy powder used as the raw material powder is different from Sample No. 5-1 produced in Test Example 1.

Sample No. 2-1 is a mixture of iron alloy powder with a composition of Fe—0.5 mass % Mo—0.2 mass % Mn and pure C powder, and the raw material powder does not contain Ni. Sample No. 2-1 in Test Example 2 is different from Sample No. 2-1 in Test Example 1 (TABLE 2).

The tensile strength (MPa) of each sintered material of the produced sample was measured in the same manner as in Test Example 1, and the results are shown in TABLE 3 and FIG. 2. FIG. 2 is a graph in which the horizontal axis represents the Ni content (Ni mass, mass %) of the entire composition of the sintered material (the entire iron-based alloy), and the vertical axis represents the tensile strength (MPa) of the sintered material.

The local concentration distribution (content) of Ni for each sintered material of the produced sample was examined in the same manner as in Test Example 1, and the results are shown in TABLE 3. In addition, when the sintered material of each sample was measured in the same manner as in Test Example 1, the relative density was 99%; the oxygen content was not more than 2000 ppm; and the entire composition was almost the same as the basic composition of the raw material powder (Fe—(value in TABLE 3) Ni—0.5 mass % Mo—0.2 mass % Mn—0.3 mass % C).

TABLE 3 Ni CONCENTRATION DISTRIBUTION Ni TENSILE (Ni AMOUNT, MASS %) SAMPLE AMOUNT STRENGTH MAXIMUM MINIMUM No. (MASS %) (MPa) VALUE VALUE 2-1 0 1016 0 0 2-2 1 1466 1 1 2-3 2 1542 2 2 2-4 3 1604 3 3 2-5 4 1654 4 4 2-6 5 1696 5 5 2-7 6 1652 6 6 2-8 7 1623 7 7 2-9 8 1587 8 8 2-10 9 1556 9 9 2-11 10 1535 10 10

The relative density is 97% or more in this test, and TABLE 3 and FIG. 2 indicate that the local Ni concentration distribution (content) influences the tensile strength of the sintered material uniformly containing Ni. The tensile strength is low without Ni (sample No. 2-1). The higher the Ni content, the higher the tensile strength. Here, the tensile strength is 1460 MPa or more when the Ni content is 1 mass % or more. In particular, when the content of Ni is more than 2 mass % but less than 8 mass %, the tensile strength is 1600 MPa or more, and the tensile strength is higher as the content approaches 5 mass %, and the tensile strength becomes the maximum (peaks) at 5 mass %. Although the reason for this is unclear, the test results indicate that for a sintered material having a relative density of 97% or more and a Ni concentration distribution of 0.2 mass % or more and less than 21 mass %, when the content of Ni in the entire composition of the sintered material is more than 2 mass % and less than 8 mass %, and further not less than 3 mass % and not more than 7 mass %, the tensile strength can be improved further and a tensile strength of 1600 MPa or more can be obtained.

TEST EXAMPLE 3

The effect on tensile strength was examined by varying the oxygen content.

Here, a sintered material was produced in the same manner as in Test Example 1, except that a degree of reduction treatment for an iron alloy powder used as a raw material powder differs from that of Sample No. 5-1 produced in Test Example 1.

For each sintered material of the produced sample, the oxygen content (oxygen content, mass %) and tensile strength (MPa) were measured in the same manner as in Test Example 1, and the results are shown in TABLE 4 and FIG. 3. FIG. 3 is a graph in which the horizontal axis represents the oxygen content (oxygen content, mass ppm) of the entire composition of the sintered material, and the vertical axis represents the tensile strength (MPa) of the sintered material.

In addition, when the local Ni concentration distribution (content) was examined for each sintered material of the produced sample in the same manner as Test Example 1, the local Ni concentration distribution was substantially the same as the local Ni concentration distribution (content) in Sample No. 5-1. In addition, the relative density of the sintered material of each sample was 99%, and the overall composition was almost the same as that of the base composition of the raw material powder (Fe—5 mass % Ni—0.5 mass % Mo—0.2 mass % Mn—0.3 mass % C) and was substantially composed of martensitic tissue.

TABLE 4 TENSILE SAMPLE OXYGEN AMOUNT STRENGTH No. (MASS ppm) (MPa) 3-1 980 1695 3-2 2000 1696 3-3 3000 1604

The relative density is 97% or more in this test, and TABLE 4 and FIG. 3 show that for sintered materials that uniformly contain Ni, the oxygen content influences the tensile strength. This test shows that the lower the oxygen content, the higher the tensile strength. If the oxygen content is 3000 mass ppm or less from TABLE 4 and FIG. 3, the tensile strength is 1600 MPa, which is considered a high strength sintered material. Furthermore, if the oxygen content is less than 3000 mass ppm, the tensile strength is not less than 1650 MPa, and further reaches about 1700 MPa, which indicates that the strength is equivalent to that of the wrought steel described above. One of the reasons for this is considered that the oxygen content was low, which reduces the oxide that could be the starting point of cracking and breaking. The test results showed that for sintered materials having a relative density of 97% or more and a Ni concentration distribution of more than 0.2 mass % and less than 21 mass %, the tensile strength can be further improved if the oxygen content of the entire composition of the sintered material is less than 3000 mass % and further less than 2000 mass %. In addition, it was shown that the oxygen content can be adjusted by removing oxygen by an appropriate reduction treatment of the raw material powder.

It should be understood that the embodiments disclosed herein are exemplary in all respects and are not restrictive in any respect. It is intended that the scope of the invention be defined by the appended claims, not by the foregoing description, and include all modifications within the meaning and scope of the claims. For example, in Examples 1 to 3 described above, the composition may be changed (for example, Mo, Mn content may be changed, or Si or B may be added) or the manufacturing conditions may be changed. 

1. A sintered material made of an iron-based alloy, wherein a content of Ni is more than 0.2 mass % and 10 mass % or less in an entire iron-based alloy; a content of C is more than 0 mass % and 2.0 mass % or less in the entire iron-based alloy; at least one element selected from Mo, Mn, Cr B and Si is more than 0 mass % and 5.0 mass % or less in total in the entire iron-based alloy; and a rest of the iron-based alloy is Fe and incidental impurities, wherein a content of Ni in a local region of the iron-based alloy is more than 0.2 mass % and less than 21 mass %, and wherein a relative density is 97% or more.
 2. The sintered material as claimed in claim 1, wherein the content of Ni in the entire iron-based alloy is more than 2 mass % and less than 8 mass %.
 3. The sintered material as claimed in claim 1, further comprising: oxygen of a content of less than 3000 mass ppm.
 4. The sintered material as claimed in claim 1, further comprising: tissue made of martensite.
 5. The sintered material as claimed in claim 1, wherein the content of Ni in the entire iron-based alloy is more than 2 mass % and less than 8 mass %, and wherein a content of oxygen is less than 3000 mass ppm.
 6. The sintered material as claimed in claim 1, further comprising: tissue made of martensite, wherein the content of Ni in the entire iron-based alloy is more than 2 mass % and less than 8 mass %, and wherein a content of oxygen is less than 3000 mass ppm.
 7. The sintered material as claimed in claim 2, wherein the content of Ni in the entire iron-based alloy is more than 4 mass % and less than 6 mass %.
 8. The sintered material as claimed in claim 3, wherein the content of oxygen is less than 2000 mass ppm.
 9. The sintered material as claimed in claim 1, wherein the content of C in the entire iron-based alloy is 0.2 mass % or more and 0.8 mass % or less.
 10. The sintered material as claimed in claim 1, wherein the content of Mo in the iron-based alloy is 0.15 mass % or more and 0.8 mass % or less, and wherein the content of Mn in the entire iron-based alloy is 0.15 mass % or more and 0.8 mass % or less.
 11. The sintered material as claimed in claim 1, wherein the content of N in the local region of the iron-based alloy is 1 mass % or more and 12 mass % or less. 